The copper head gasket spray has a good reputation, it can seal a small gap, plus distributes the heat more evenly on the gasket-to-head interface. Good stuff to use in my opinion.
Permatec Copper Spray In this situation I would definitely use a new gasket from a genuine Packard source, they will compress whereas an old one refitted will not always seal as reliably (- although they often do, but you are taking a bit of a risk).
I guess you tightened the head nuts down in sequence as per the Service Manual(?), so it might be a leak via a crack around a stud or elsewhere. Get other opinions, but IMO how and when you did the re-torquing is not likely to be the issue in this case, you seem to have done the right thing. Just don't keep torquing in an attempt to fix the leak as you might break or strip a stud.
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